Underwater Vehicle Bouyancy System

ABSTRACT

A multiple stage buoyancy changing system, or variable buoyancy device, for an underwater vehicle. The multiple stage buoyancy changing system comprises: a pressure hull containing a flexibly-sized internal fluid reservoir; a flexibly-sized external fluid reservoir attached to the pressure hull and connected to the internal reservoir; a system of devices and channels configured to move fluid between the internal fluid reservoir and the external fluid reservoir to change a displaced volume of the vehicle. Each stage of the variable buoyancy device can be optimized for maximum energy efficiency while changing the vehicle&#39;s displaced volume within an ambient pressure range. A control system for the variable buoyancy device engages different stages depending on ambient external pressure such that maximum energy efficiency is achieved over a large range of pressures/depths.

INTRODUCTION

This application claims priority to U.S. Provisional Patent Application Ser. No. 61/309,420, filed Mar. 1, 2010, titled Underwater Glider, and is a continuation of U.S. patent application Ser. No. 12/890,584, filed Sep. 24, 2010, titled Autonomous Underwater Vehicle, the disclosures of which are incorporated herein by reference in their entireties.

The present teachings relate to a multiple stage buoyancy changing system for underwater vehicles.

BACKGROUND

Underwater vehicles that are propelled by changes in buoyancy have become commercial in recent years and have demonstrated the ability to operate at sea for long periods. Such vehicles, including underwater gliders, are in use, and continue to be developed, for oceanic research, coastline monitoring, and other applications. Existing designs for a buoyancy changing system, or “buoyancy engine,” are typically specialized to perform energy efficiently in limited ranges of depths, or pressures, such as, for example, either for water less than 200 meters deep or for water deeper than 200 meters. This can be a function of the hydrostatic pressure surrounding the vehicle, which is approximately linearly related to depth. The energy-efficient operation of existing underwater vehicles is therefore limited to a particular band of underwater depths.

Underwater vehicles can work, for example, as described in U.S. Pat. No. 3,157,145 to Farris et al., the entire disclosure of which is incorporated herein by reference. An underwater vehicle can comprise an adjustable portion such as an external reservoir for changing the volume displacement of the vehicle. The external reservoir can initially be filled with a fluid, such as oil, to maximize the vehicle's buoyancy when it is initially launched in the water. A valve can be controlled, allowing fluid to escape the external reservoir to an internal storage reservoir. As fluid leaves the reservoir, the volume displacement of the vehicle decreases while its mass stays the same, causing an increase in density of the vehicle relative to the surrounding water. This density difference causes a downward force causing the vehicle to descend in the water, for example along a desired dive profile path.

When the vehicle has reached the deepest point of its desired path, a pump system is used to move fluid from the internal reservoir back out to the external reservoir. This causes the vehicle to be less dense than the surrounding water and rise back to the surface under positive buoyancy. As the vehicle descends or ascends (also referred to as diving and climbing, respectively), if it has wings such as those typically provided on underwater gliders, it can move forward.

If the pump system employed in an underwater vehicle is not energy efficient in both shallow and deep water, then dives to different depths can use far more energy than just diving to the depth (or pressure) at which the system is most efficient.

SUMMARY

The present teachings provide a multiple stage buoyancy changing system, or variable buoyancy device, for an autonomous underwater vehicle. The system or device comprises: a pressure hull; a flexible sized internal reservoir configured to hold a fluid; and a flexible sized external reservoir, or bladder, connected to the internal reservoir via one or more channels, each channel having multiple valves and pumps. The one or more channels are configured to exchange fluid between the reservoirs as variable buoyancy device stages which are specifically optimized for energy efficiency at a range of ambient external pressures for multiple segments of a dive profile.

The first segment of the dive profile can be handled by a first stage of the variable buoyancy device, the second segment of the dive profile can be handled by the second stage of the variable buoyancy device, et cetera, up to an Nth stage corresponding to a maximum depth or pressure to which the vehicle is designed to dive.

The present teachings also provide a method for controlling a multiple stage buoyancy changing system, or variable buoyancy device, for an autonomous underwater vehicle having an external pressure sensor and a volume measurement system for either, or both, of an internal reservoir and an external reservoir. The external pressure sensor and the volume measurement system are connected to an internal processor and electronics that control the valves and pumps of the variable buoyancy device. The present teachings include logic for selecting which stage of the variable buoyancy device is used at any given depth.

The present teachings further provide a multiple stage variable buoyancy device for an autonomous underwater vehicle that includes a pump and motor combination with a continuous variable transmission that can electronically adapt to a torque-speed curve to rapidly obtain an optimal pumping rate for changing buoyancy.

Additional objects and advantages of the present teachings will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present teachings. The objects and advantages of the teachings will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present teachings, as claimed.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present teachings and, together with the description, serve to explain the principles of the teachings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary path of an autonomous underwater vehicle such as a glider descending and ascending through multiple depth ranges.

FIG. 2 schematically illustrates an exemplary embodiment of a multiple stage fluid pump and valve system for an underwater vehicle in accordance with the present teachings, the system being optimized for one to “N” stages to be energy efficient at all ranges of pressure (or depth) underwater.

FIG. 3 illustrates an exemplary embodiment of a decision scheme used by an electronic control system for a multiple stage variable buoyancy device (VBD), wherein each stage is designed to pump at a particular output pressure range for maximum energy efficiency throughout an underwater dive profile. The illustrated decision scheme uses information from an external pressure sensor and an internal volume sensor to know if a target volume has been achieved. Other system of the underwater vehicle can determine whether a change in vehicle volume is needed.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to embodiments of the present teachings, examples of which are illustrated in the accompanying drawings.

In many autonomous underwater vehicles, most of the vehicle's energy is used to change buoyancy by pumping fluid of low compressibility into an external reservoir from an internal reservoir. The amount of pressure required to pump fluid into the external reservoir varies with external ambient pressure, which is approximately linearly related to depth. For example, in shallow water the required pressure can have a magnitude of a few hundred pounds per square inch (psi), whereas in deeper water it can have a magnitude of several thousand psi.

The difference in pressures required to pump fluid into the vehicle's external reservoir creates a design dilemma, because pump and motor combinations that are powerful enough to pump fluid at high pressure for deeper water are not energy efficient for pumping at lower pressures needed in shallower waters. Lower pressure pumps and motors that are more energy efficient in shallow water cannot provide sufficient pumping pressure in deeper waters. This design dilemma typically causes existing autonomous underwater vehicles to be energy efficient for a limited range of depths. While they may function over a wide range of depths, their energy efficiency is not optimal outside of a certain optimum depth range.

The present teachings provide a multiple stage buoyancy changing system, or variable buoyancy device, that can make an underwater vehicle energy efficient over a large range of depths. Multiple stages including a channel, a pump, a motor, and a valve can be optimized to each cover a portion of an external pressure range that the vehicle will encounter in a typical dive cycle. By sensing the depth or ambient pressure surrounding the underwater vehicle in a given dive profile, and engaging the correct stage for pumping fluid for that ambient pressure, a system in accordance with the present teachings allows a single autonomous underwater vehicle to produce energy efficient vertical motion covering a broad range of depths, including shallow coastal waters to deep ocean domains.

FIG. 2 schematically illustrates an exemplary embodiment of a multiple stage buoyancy system in accordance with the present teachings. By utilizing multiple channels within the device, including bypass channels, a multiple stage buoyancy system of the present teachings achieves high efficiency in buoyancy changes without allowing one stage to compromise or restrict the performance of any other stage. Check valves can be provided to prevent fluid from returning to previous pump stages. A filter is shown, which can protect the system from contaminants in the fluid that would decrease the flow or clog the valves, but is not essential to operation.

The present teachings contemplate an underwater vehicle including as many stages as are deemed necessary and expedient to produce the best trade-off between energy use for pumping and (a) the mass of parts needed for each stage, (b) the volume occupied by each stage within the pressure hull, and (c) the complexity of controls and plumbing.

FIG. 1 illustrates an exemplary dive profile having one or more distinct depth (or pressure) ranges for which different variable buoyancy device pump stages are utilized. The choice of which pump stage to utilize can be made, for example, by the vehicle's on-board processor with input from various sensors and command files. In accordance with certain embodiments, the autonomous underwater vehicle can abort a dive when a problem (e.g., a system error) is detected. When a dive is aborted, the autonomous underwater vehicle can, for example, pump as much fluid into the external reservoir as possible to reach the surface for retrieval, preferably using the most efficient stages of the variable buoyancy system.

The fluid displacement systems (e.g., the pump, motor, and valve systems) of the present teachings need not be of the same type. Different stages can comprise different components. Exemplary fluid displacement systems that can be used in accordance with the present teaching include, for example, a piston-driven pump, a systolic pump, a Stirling engine, and/or other suitable devices that can move fluid.

A multiple stage buoyancy system in accordance with the present teachings can be implemented using a variety of approaches that embody the principle of depth/pressure dependent selection of the most efficient pump stage. An example of an implementation and decision process is illustrated in FIG. 3. By using a pressure sensor that detects the surrounding water pressure at a given depth, and a volume sensor that detects the vehicle's displacement volume, the control system and/or electrical logic of the vehicle can enable a pumping stage that is most energy efficient for the detected environmental pressure if a change in external volume is needed. In principle, a system of many (“N”) stages can be employed, wherein two is the simplest case and may be adequate for many different vehicles. The present teachings illustrate in FIG. 2, however, that more than two stages can be utilized to achieve high efficiency across the entire depth of the ocean.

Another embodiment of the present teachings contemplates utilizing a pump and motor in combination with a continuous variable transmission that can adapt to a torque and speed curve, resulting in different pumping rates at different depths to efficiently change the buoyancy of an autonomous underwater vehicle. Continuously variable transmissions can provide an effective continuum of torque-speed ratios over a predetermined range, with slower speeds corresponding to higher torque output and higher speeds corresponding to lower torque output

The illustrated exemplary embodiment of FIG. 2 places the stages of the multiple stage buoyancy system in parallel with each other to eliminate a potential negative impact of serial placement. Serial placement can impede optimal performance by restricting the downstream pump's access to the internal reservoir.

Certain embodiments of the present teachings can combine two or more stages to increase the rate of pumping and thus change of buoyancy.

During an underwater vehicle's descent, fluid can move from the external reservoir to the internal reservoir when a high-pressure return valve between the external reservoir and the internal reservoir is opened. Ambient pressure can be used to push fluid from the external reservoir to the internal reservoir by pressing on the external reservoir. In addition, the autonomous underwater vehicle can have a reduced internal pressure (e.g., a vacuum) that encourages fluid flow from the external reservoir to the internal reservoir. Certain embodiments of the present teachings also contemplate using one or more pumps to drive fluid from the external reservoir to the internal reservoir if more speed is required in that process.

In accordance with certain embodiments of the present teachings, a connection can exist from the output of one pump stage to the intake of another pump stage. This series-like plumbing can function as a safety path for any pump stage that needs priming.

An autonomous underwater vehicle employing control and buoyancy systems in accordance with the present teachings can travel long distances (e.g., thousands of kilometers) over durations of many months using buoyancy changes that combine algorithms and multiple stage buoyancy control to conserve onboard stored energy by utilizing an optimized fluid displacement strategy, selecting the most energy efficient fluid displacement mechanism(s) to traverse all desired diving profiles.

Other embodiments of the present teachings will be apparent to those skilled in the art from consideration of the specification and practice of the teachings disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the present teachings being indicated by the following claims. 

1. A multiple stage buoyancy changing system for an underwater vehicle, the multiple stage buoyancy changing system comprising: a pressure hull containing a flexibly-sized internal fluid reservoir; a flexibly-sized external reservoir connected to the pressure hull; and a system of devices and channels that connect and move fluid between the internal fluid reservoir and external fluid reservoir to change a displaced volume of the underwater vehicle.
 2. The multiple stage buoyancy changing system of claim 1, wherein there are enough stages to operate over any range of water depth or pressure that the pressure hull can withstand.
 3. The multiple stage buoyancy changing system of claim 1, wherein there are enough stages to minimize the energy used to change a buoyancy of the vehicle any range of water depth or pressure that the pressure hull can withstand.
 4. The multiple stage buoyancy changing system of claim 3, wherein each stage is optimized to operate for a particular range of pressure output.
 5. The multiple stage buoyancy changing system of claim 1, wherein the system of devices and channels comprises at least one motor, at least one gear train, at least one coupling, at least one pump, and at least one valve.
 6. The multiple stage buoyancy changing system of claim 1, further comprising a control system that uses sensors to determine if a change in displaced volume is necessary.
 7. The multiple stage buoyancy changing system of claim 1, wherein the underwater vehicle has a front and a rear, and is configured to shift its center of buoyancy and center of mass toward the front or the rear while decreasing and increasing its buoyancy, respectfully.
 8. The multiple stage buoyancy changing system of claim 1, wherein the stages of the system are controlled by electronics and/or software that can detect and use external pressure measurements in a decision process.
 9. The multiple stage buoyancy changing system of claim 1, wherein one of the channels comprises an electronically controlled valve that can be selectively opened to allow fluid to pass from the external fluid reservoir to the internal fluid reservoir under the influence of only external pressure.
 10. The multiple stage buoyancy changing system of claim 4, further comprising a continuous variable transmission that can electronically adapt to a torque-speed curve to obtain an optimal pumping rate for changing displaced volume. 